

1891. ] Rocky Mountain Rhizopods. 1083 
Sometimes, however, some of the investing elements can be 
seen which must have been directiy deposited on the shell from 
the outside,—inorganic particles and diatoms which are much too 
large ever to have gone through the mouth of an individual 
belonging to the species.‘ At other times, again, the whole shell 
seems to have been formed by external apposition, as in many 
specimens of Pseudodifflugia hemispherica which I have exam- 
_ ined, in which nearly the whole of the shell was made of dia- 
toms, still containing their plasma and their yellowish or brown 
chromatophores. 
Before concluding, I would emphasize the constant dislike of 
Rhizopods for limestone. Not only are limestone countries 
always poor in Rhizopods, as Leidy showed in 1879, but even 
species that easily endure the presence of lime will never use any 
particle of it for the building of their shells. In the spring of this 
year I examined numerous species of Difflugia which inhabited 
the muddy bottom of Geneva. This mud, under the microscope, 
is seen to be composed of a mixture in nearly equal proportions 
of very fine particles of quartz and transparent limestone. A 
little chip of limestone, two- or three-thousandths of a millimeter 
in diameter, often very closely resembles another such chip of 
quartz. Without a careful examination a professional observer 
might easily be deceived ; but a Difflugia is not, and will always 
choose quartz“ particles for the building materials of its shell.’ 
t Indeed, in some forms ee eroi ifera, Dif. rubescens) this seems to be the 
only way in which the shell can be b 
5 Besides these Rhizopods, I have found, of course, mixed with them, many other 
organisms, ae soria, Flagellata, Rotifers, Nematodes, etc. Of these I will cite only 
Jenodinium cinctum and Peridinium tabulatum, and three Heliozoa, 

Actinophrys sol, Hiter ophrys ——? Acanthocystis myriospina Penard? (Acant 
ith simple spines, Leidy) ; fine specimens of this latter were abundant at 12,coo feet 

